High purity iron finds a number of applications including, among others, the following:
(1) Powder Metallurgy. By means of this technique metallic parts are formed by pressing iron powder. This is particularly useful when the form of said parts poses great difficulty for their fabrication by conventional foundry and machining methods.
(2) Fabrication of welding electrodes. In this industry iron powder of controlled size and chemical composition is required as a raw material to form the electrode's skin.
(3) Fabrication of brake parts for vehicles.
(4) As chemical reagent for copper precipitation.
(5) In the pharmaceutic industry, in minor quantities.
The methods currently employed for producing high purity iron present the following disadvantages:
(a) Atomizing of pure molten iron by means of a gas jet. Although this method provides iron of adequate purity, its operation is difficult and expensive because it involves controlled melting of iron and very specialized equipment and labor for such atomizing.
(b) Electrolytic Processes. These processes are very expensive and are employed only for very special cases. The use of such processes is not economical for the massive industrial production of iron powder.
(c) Direct Reduction of iron ores by means of reducing gases (usually mixtures of hydrogen and carbon monoxide), followed by grinding to produce the desired particle size. This method has the disadvantage that iron ores contain appreciable quantities of gangue and undesired elements, such as sulfur and phosphorus which present difficulties for their elimination and interfere with some applications of the product, apart from the cost involved in producing the reducing gases.
(d) A substantial amount of the world production of high purity iron in powder form is produced through the Hoganas Process in Sweden and the United States.
In the Hoganas Process particles of iron ore concentrate are reduced with coke as reductant, and limestone is added for sulfur removal and to prevent the reduced material from sticking to the walls of a crucible or "sagger" made of ceramic material, by forming a layer of limestone or coke between the charge and the wall of the crucible.
The crucibles are taken into a furnace wherein they are subjected to high temperatures, in the range of 1000.degree. to 1200.degree. C. The furnace is circular. It is divided into zones where the temperature is varied by setting on and off the burners associated to each zone.
When the iron ore has been reduced, it is cooled in a non-oxidant atmosphere to avoid reoxidation of metallic iron, and then the crucibles are discharged. This iron must then be separated from the other elements: ashes from coke, limestone and others, and then it must be ground, if necessary, to the desired particle size.
This process is complicated and requires excessive handling of crucibles and materials. Furthermore, handling of crucibles causes deterioration and breakage thereof with the corresponding replacement costs.
(e) A modification to Hoganas Process is the process developed by Ontario Research Foundation, which also employs ceramic crucibles carefully filled with iron ore and coke so that coke forms a layer on the walls to prevent the reduced material from sticking thereto.
The filled crucibles are then mounted on cars moving through a tunnel furnace having heating, reducing and cooling zones, and exit already cooled at the other end of the furnace. Iron is separated and ground to the desired particle size.
This process also presents disadvantages and complications because of the moving cars through zones at very high temperatures, and the excessive handling and movement of crucibles and cars.
(f) In order to decrease the gangue and carbon content in the produced iron, the PYRON Process uses "mill scale" as raw material, instead of iron ore concentrate. Mill scale is the waste generated in rolling mills and corresponds to the oxide layer of ingots which is removed by action of the mill rolls. This "mill scale" is produced in large quantities in all lamination plants, and is mainly iron oxide free of undesired elements.
Mill scale is dried and heated to about 900.degree. C. and then ground to particle sizes smaller than 0.15 mm. This material is then reduced with hydrogen in a continuous conveyor which passes through a furnace at a temperature of about 1000.degree. C. The reduced product in form of sintered lumps in then ground in a ball mill to the desired size.
This process involves high operating and investment costs because it requires production of hydrogen and also uses a moving conveyor at high temperatures.
(g) It has also been proposed to reduce mill scale be means of hydrogen in a fluidized bed reactor. This type of process presents the disadvantages of requiring hydrogen and also a very strict control in order to maintain fluidization and homogeneity of the product.